Most of the work will deal with transketolase from two sources. The yeast enzyme shows cooperatively related to dimerization and conformational changes, both induced by thiamin pyrophosphate which is also the coenzyme for the catalytic process. Two separate Km values are measured for the catalytic process (Km) and two for the cooperativity (K tau), none of the four values being equal to any of the others. Competitive inhibitors will be used to determine whether the allosteric sites are the same as the catalytic sites and are affected equally and whether they may actually be the same sites. The study will be carried out by analysis of the lag in reactivation and the steady-state velocity seen in the progress curves of the reaction. The allosteric effect (dimerization) will also be studied by ultracentrifugation. Transketolase will be purified from human erythrocytes. The kinetic mechanism and possible allosteric effects will be investigated. An inhibitor of transketolase is present in the hemolysates. The possible physiological significance will be investigated and an attempt will be made to purify it so that its nature may be determined.